Polyimide composite tube

ABSTRACT

A seamless polyimide composite tube which includes a polyimide layer, a conductive primer layer on the surface of the polyimide layer and a fluororesin layer on the surface of the primer layer and a method of manufacturing the composite tube.

This application is a continuation of U.S. application Ser. No.08/531,297 filed Sep. 20, 1995, now U.S. Pat. No. 5,582,886 which is acontinuation of Ser. No. 08/167,360, filed December 16, 1993, nowabandoned.

FIELD OF THE INVENTION

The invention relates to a seamless polyimide composite tube whichcomprises a polyimide layer, a conductive primer layer on the surface ofthe polyimide layer and a fluororesin layer on the surface of the primerlayer. The invention also relates to a method of manufacturing thepolyimide composite tube.

BACKGROUND OF THE INVENTION

Heat-resistant resins, which have excellent mechanical and chemicalproperties, have been molded into many different shapes such as films,tubes, rods, formed materials, coating materials, etc., and used asflexible printed substrates, heat-resistant electric wire insulatingmaterials, magnetic tapes, and the like. Future additional uses of theheat-resistant resins are also likely to become apparent. The heatresistant resins are used typically as revolving and delivering belts orconveyor belts for light electrical sounders, or heat fixing belts forcopiers, laser beam printers, and the like.

As a heat-fixing member of a fixing apparatus in order to fix tonerdeveloped on copying paper or decalcomania paper, a heat roller isgenerally used in copiers or laser beam printers that utilizeelectrophotographic technology. More specifically, the papers developedwith toner are passed through an opening between a fixing roller havinga heating mechanism (heat roller) and a pressure roller, one pagefollowing another, thereby heating, melting and then fixing the toner onthe paper.

There has been research on the use of a polyimide tube, instead of theheat roller, for a fixing apparatus. The inside polyimide tube isequipped with a live roller, a tension roller and a heater. Copyingpaper developed with toner is supplied one after another to an openingbetween the polyimide tube and a backing-up roller, thus fixing toner onthe paper. This fixing apparatus heats and fixes the toner at thesurface of the polyimide tube via the heater. Therefore, different fromthe heat roller, this polyimide tube does not require time for heatingitself, and can start fixing toner on paper as soon as the power supplyswitch of the fixing apparatus is turned on. In addition, the capacityof the heater used for this fixing apparatus is small, and the apparatusconsumes little electricity.

When this fixing apparatus is used, the toner developed on copying paperis instantaneously melted and fixed on the paper by the heater employedinside the polyimide tube. Therefore, if the polyimide tube wallthickness is uneven, the toner cannot be melted evenly. As a result, anundesirable offset phenomenon occurs. In this sense, it is necessary tominimize any uneveness in the polyimide tube wall thickness much aspossible.

When the polyimide tube having uneven internal diameters in alongitudinal direction is rotated by two or three rollers, the tubemeanders in a longitudinal direction. Therefore, when the tube is usedas a heat-fixing seamless belt, the tube is required to have precisecylindricity.

One example of a method of manufacturing polyimide tubes with uniformtube wall thickness is disclosed in Japanese Published Unexamined PatentApplication No. Sho 62-19437. The polyimide tube is manufactured in thefollowing steps:

pouring polyamic acid solution into a molding pipe such as a glass pipe,stainless pipe, or the like with a smooth internal surface;

holding the molding tube in a vertical position;

dropping a bullet-like object through the solution by its own weight,thereby forming a hole inside the solution;

heating and drying the solution inside the molding pipe, thus causing itto become imide by imide reaction and forming a tube; and

extracting the tube from the molding pipe.

The inventors of the present invention also disclose another method inJapanese Published Unexamined Patent Applications No. Hei 3-180309 andNo. 3-261518. In this method, a polyimide precursor solution such aspolyamic acid solution is coated on the outside surface of a core. Thesolution on the core is then heated and dried, thus causing it to becomeimide by imide reaction and forming a tube. Finally, the tube isseparated from the core.

However, since only an extremely thin tube can be formed by the methodof Japanese Published Unexamined Patent Application Sho 62-19437, thetube has to be laminated repeatedly by repeating the forming, drying andheating steps of this method many times. It is also extremely difficultto extract the tube from the inside surface of the glass or stainlesspipe. Since the polyimide tube is extracted from the inside of the pipe,a long polyimide tube with a small inside diameter can hardly be made.Moreover, when the polyimide tube is used as a fixing means for anelectrophotographic printer or a laser printer fix toner developed oncopying papers at the tube surface via the heater, the tube's propertiesof separating the toner from itself are so critical that offset occurs.In other words, toner left on the tube is later printed on copying paperby the rotation or the tube, thus staining both the paper and the tubesurface. Since the polyimide tube is also likely to generate staticelectricity, the toner, right before its fixation, is repulsed by thestatic, thereby blurring copy image and weakening resolution.

Similar to the above-noted problems, Japanese Published UnexaminedPatent Applications No. Hei 3-180309 and No. Hei 3-261518 have problemsof poor separation properties as well as static electricity of thepolyimide tubes.

SUMMARY OF THE INVENTION

In order to solve the problems of conventional methods, the inventionprovides an anti-static seamless polyimide composite tube, whichcomprises a polyimide resin layer as a substrate and whose surface hasgood separation properties against toner; and a method of efficientlymanufacturing the seamless polyimide composite tube.

In order to accomplish the above, the polyimide composite tube of theinvention comprises a seamless layer, which contains polyimide as one ofthe principal ingredients, as a substrate. The outside surface of theseamless layer is coated with a conductive primer layer; the outsidesurface of the conductive primer layer, in addition, is coated with abaked fluororesin layer. Thus, the polyimide composite tube is formed.

It is preferable in this composition that the polyimide is contained inthe seamless layer at about 90 mol %. Copolymerized polyamide can becontained in the layer up to about 10 mol %.

It is preferable in this composition that the seamless polyimidecomposite tube of the invention is manufactured as follows:

coating a polyimide precursor solution on the outside surface of ametallic cylinder;

casting the precursor solution with a metallic ring at a uniformthickness;

drying and heating the solution, thus forming a half-hard polyimidelayer by a midway imide reaction;

coating a conductive primer layer on the surface of the half-hardpolyimide layer;

coating a fluororesin layer on the surface of the conductive primerlayer; and

heating the half-hard polyimide layer coated with the conductive primerlayer and the fluororesin layer to complete the imide reaction as wellas to bake the fluororesin layer at the same time.

As used herein, uniform thickness refers to a thickness within ±15%,preferably ±12.5%, or ideally ±10% of 3-500 μm for a preferablethickness of the polyimide layer over the entire length of the layer.

It is preferable in this composition that the thickness of the polyimidelayer (substrate) is from 3 μm to 500 μm.

It is also preferable in this composition that the conductive primerlayer contains at least one compound selected from the group consistingof polyphenylenesulfide; polyethersulfone; polysulfone; polyamideimide;polyimide; derivatives of polyphenylenesulfide, polyethersulfone.polysulfone, polyamideimide, or polyimide; and fluororesin.

It is further preferable in this composition that the thickness of theconductive primer layer is from 0.5 μm to 10 μm and that the layercomprises an exposed area.

It is preferable in this composition that the surface electricresistance of the conductive primer layer is from 1×10⁻² Ω·cm to 1×10⁷Ω·cm.

It is also preferable in this composition that the conductive primerlayer contains 1-40% by weight of carbon powder.

It is preferable in this composition that the fluororesin is at leastone compound selected from the group consisting ofpolytetrafluoroethylene (PTFE),tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), andtetrafluoroethylene-hexafluoropropylene copolymer (FEP).

It is also preferable in this composition that the thickness of thefluororesin layer is from 2 μm to 30 μm.

It is further preferable in this composition that the fluororesin layercontains 0.1-3.0% by weight of carbon powder.

The method of manufacturing a seamless polyimide composite tube of theinvention comprises the following steps:

coating a polyimide precursor solution on the outside surface of ametallic cylinder;

casting the precursor solution with a metallic ring at a uniformthickness;

drying and heating the solution, thus forming a half-hard polyimidelayer by a midway imide reaction;

coating a conductive primer layer on the surface of the half-hardpolyimide layer;

coating a fluororesin layer on the surface of the conductive primerlayer; and

heating the half-hard polyimide layer coated with the primer layer andthe fluororesin layer to complete the imide reaction as well as to bakethe fluororesin layer at the same time.

It is preferable in this method that a reduction ratio of the thicknessof the polyimide layer at the midway imide reaction (before thecompletion of the imide reaction) is 50-95%. The reduction ratio iscalculated by the following formula:

    x={(V.sub.o -V.sub.a)/V.sub.o }×100,                 Formula 1

wherein x represents reduction ratio of the thickness of the polyimideprecursor layer;

V_(o) represents the original thickness of the polyimide precursorsolution right after the solution is coated on the metallic cylinder;and

V_(a) represents the thickness of the half-hard polyimide layer at themidway imide reaction.

The midway imide reaction of the half-hard polyimide layer can also beconfirmed by an infrared absorption spectrum analysis (IR). Atitrimetric analysis of a --COOH group, --NH₂ group, and a --NH groupcan also determine if the polyimide layer is half-hard.

It is also preferable in this method that the fluororesin is at leastone compound dispersed in water (compound dispersion), and that thecompound is selected from the group consisting ofpolytetrafluoroethylene (PTFE),tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), andtetrafluoroethylene-hexafluoropropylene copolymer (FEP).

It is further preferable in this method that the fluororesin is coatedon the surface of the primer layer by a dipping and holding method.

It is preferable in this method that the fluororesin is mixed withcarbon black.

It is preferable in this method that the conductive primer is coated onthe surface of the half-hard polyimide layer, which is formed on thesurface of the metallic cylinder, and the fluororesin is then coated onthe surface of the conductive primer layer.

It is also preferable in this method that the fluororesin is coatedwhile the metallic cylinder is rotated.

It is preferable in this method that the polyimide precursor solution isan aromatic polyimide precursor solution, and that the viscosity of thesolution is 50-10000 poise.

It is also preferable in this method that the thickness of the polyimideprecursor solution coated on the metallic cylinder is 10-1000 μm.

It is preferable in this method that a metallic mold is arranged outsideof the metallic cylinder at a fixed distance, and that at least one ofthe metallic cylinder and the metallic mold is shifted, thereby castingthe polyimide precursor solution at a uniform thickness.

As used herein, uniform thickness refers to a thickness within ±15%,preferably ±12.5%, or ideally ±10% of 3-500μm for a preferable thicknessof the polyimide layer over the entire length of the layer.

It is also preferable in this method that the conductive primer layercontains 1-40% by weight of carbon powder.

It is further preferable in this method that the conductive primersolution is coated on the half-hard polyimide layer while the metalliccylinder is rotated.

One example of an apparatus for manufacturing the polyimide compositetube of the invention comprises the following:

a component for casting a polyimide precursor solution coated on thesurface of a metallic cylinder at a uniform thickness;

a component for heating the polyimide precursor solution to make it dryor half-hard;

a component for coating conductive primer on the surface of thehalf-hard polyimide precursor layer;

a component for coating fluororesin; and

a heating component to complete the imide reaction and to bake thefluororesin at the same time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram, showing manufacturing processes of a half-hardpolyimide precursor layer of one embodiment of the invention.

FIG. 2 is a flow diagram, showing manufacturing processes of a polyimidecomposite tube of the embodiment.

FIG. 3 shows a cross-sectional view of a polyimide composite tube of theembodiment.

FIG. 4 shows a perspective view of a polyimide composite tube of theembodiment.

FIG. 5 shows a cross-sectional view of a fixing apparatus of anelectrophotographic printer employing a polyimide composite tube of theembodiment.

DETAILED DESCRIPTION OF THE INVENTION

When the polyimide composite tube of the invention comprises a seamlesspolyimide layer as a substrate, a conductive primer layer on the outsidesurface of the polyimide layer, and a baked fluororesin layer on theoutside surface of the primer layer, the polyimide composite tube isanti-static and seamless. In addition, the surface of the polyimidecomposite tube has excellent separation properties (properties ofseparating toner from the tube). The adherence between the fluororesinlayer and the polyimide layer is significant enough to keep the tubetogether and to avoid the problems of separating each layer of the tube,or the like. In this sense, the polyimide composite tube of theinvention can be practically used as a fixing means for anelectrophotographic printer or a laser printer.

When the polyimide composite tube of the invention is formed by an imidereaction after heating and drying a polyimide precursor layer casted ata uniform thickness, the polyimide composite tube has a more uniformthickness. It can thus be used as a practical fixing means for anelectrophotographic printer and a laser printer.

When the composition has a polyimide layer (substrate) having athickness of 3-500 μm, the polyimide composite tube of the inventionbecomes even stronger.

The adherence between the fluororesin layer and the polyimide layer(substrate) becomes significant if the conductive primer layer comprisesat least one compound selected from the group consisting ofpolyphenylenesulfide; polyethersulfone; polysulfone; polyamideimide;polyimide; derivatives of polyphenylenesulfide, polyethersulfone,polysulfone, polyamideimide, or polyimide; and fluororesin. Materialsdisclosed in Japanese Published Examined Patent Application No. Sho53-33972 can be used as a material for the conductive primer layer.

If the thickness of the conductive primer layer is 0.5-10 μm, theadherence among the polyimide layer (substrate), the conductive primerlayer, and the fluororesin layer improves further. If at least one edgeof the polyimide composite tube exposes the conductive primer layersurface, electrostatic charge can be preferably discharged.

When the surface electric resistance of the conductive primer layer isfrom 1×10⁻² Ω·cm to 1×10⁷ Ω·cm, the layer becomes significantlyconductive.

When the conductive primer layer of the composition contains 1-40% byweight of carbon powder, the polyimide composite tube of the inventionbecomes more practical. In addition, carbon, gold, silver, aluminum,stainless steel powder, or the like can be contained in the conductiveprimer layer.

The separation properties of the polyimide composite tube against tonerare particularly high, when the fluororesin is at least one compondselected from the group consisting of polytetrafluoroethylene (PTFE),tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), andtetrafluoroethylene-hexafluoropropylene copolymer (FEP). For example, bydipping and holding a metallic cylinder formed with a half-hardpolyimide layer and a conductive primer layer in a solution (fluororesindispersed in water) and baking the layer, a fluororesin layer with aflat and smooth surface can be preferably coated on the metalliccylinder.

A more preferable level of endurance for the polyimide composite tubecan be obtained, if the thickness of the fluororesin layer is 2-30 μm.

It is preferable that the fluororesin layer contains 0.1-3.0% by weightof carbon powder so that no electrostatic offset is generated.

As described above, the seamless polyimide composite tube of theinvention is manufactured by coating fluororesin on the surface of thehalf-hard polyimide layer in the middle of the imide reaction, and thenheating the layer to complete the imide reaction and baking thefluororesin layer at the same time. As a result, a seamless polyimidecomposite tube which comprises a polyimide layer, a conductive primerlayer and a fluororesin layer can be efficiently manufactured. Morespecifically, by not separating the treatments of completing the imidereaction and baking the fluororesin, thermal efficiency is improved, andthe time required for these treatments can also be shortened. Moreover,in this composition, the polyimide and fluororesin can be stronglybonded together to form a single unit.

With a 50-95% reduction ratio of the thickness of the polyimideprecursor, the handling of the polyimide precursor is easy enough toensure efficient manufacture of the polyimide composite tube. When thesolid part of the polyimide precursor is about 20% by weight of theprecursor, the reduction ratio of the thickness is preferably about70-95%. If the solid part is high, for example about 30-40% by weight ofthe polyimide precursor, about a 50-95% reduction ratio is practical inthe invention.

The fluororesin is at least one compound dispersed in water (compounddispersion). The compound is selected from the group consisting ofpolytetrafluoroethylene (PTFE),tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), andtetrafluoroethylene-hexafluoropropylene copolymer (FEP). Based on thiscomposition, a heat-resistant seamless polyimide composite tube withexcellent toner separation properties can be manufactured in theinvention. By coating the fluororesin by a dipping and holding method, afluororesin layer with a flat and smooth surface can be formed at auniform thickness only for the area where such coating is needed. Thisdipping and holding method also does not require a masking method, orthe like.

By mixing carbon black into the fluororesin, the seamless polyimidecomposite tube becomes anti-static.

By coating the fluororesin on the surface of a half-hard polyimidelayer, the handling of the tubes is made easy for mass production, anddust is not likely to adhere to the tubes.

When the fluororesin is coated on a rotating metallic cylinder, thefluororesin can be coated at a uniform thickness.

A highly heat-resistant seamless polyimide layer can be manufactured ifthe polyimide precursor solution is an aromatic polyimide precursorsolution. If the viscosity of the precursor solution is from 50 poise to10000 poise, the seamless polyimide layer can be cast at a more uniformthickness.

If the thickness of polyimide precursor solution coated on the metalliccylinder surface is 10-1000 μm, the thickness of the polyimide seamlesscomposite tube of the invention becomes preferable.

If a metallic mold is placed outside of a metallic cylinder at a fixeddistance, and at least one of the metallic mold and the metalliccylinder is shifted to mold a polyimide precursor solution at a uniformthickness, a seamless polyimide layer can be efficiently molded at auniform thickness.

A polyimide seamless composite tube, in which each layer is adhered inone body, can be manufactured by coating the primer and then thefluororesin on a half-hard polyimide precursor layer.

As noted above, one example of a manufacturing apparatus comprises amechanism for coating polyimide precursor solution on the surface of ametallic cylinder at a uniform thickness, a mechanism for heating thepolyimide precursor solution to make it dry or half-hard, a mechanismfor coating fluororesin on the surface of a half-hard polyimide layer,and a mechanism for heating the layers to complete the imide reactionand bake the fluororesin layer at the same time. This apparatus canefficiently manufacture a seamless polyimide composite tube.

The imide reaction of a polyimide precursor solution can be completed bygradually increasing the temperature of the solution in consideration ofthe solvent evaporation and the time required for the imide reaction,and finally by heating up the solution nearly to a glass-transitiontemperature of the polyimide precursor solution. The roles of thesolvent in the manufacturing method of the invention are as follows:

controlling the viscosity of the polyimide precursor solution;

permitting the polyimide reaction;

dissolving a polyimide precursor solution and keeping the chemicalstability of the precursor solution; and

being vaporized at a temperature below 300° C.

Such solvent includes N-methyl-2-pyrrolidone; N, N-dimethylacetamide;phenol; o-, m-, p-chlorphenol; o-, m-, p-bromphenol;2-chlor-4-hydroxytoluene; 2-chlor-5-hydroxytoluene;3-chlor-6-hydroxytoluene; and the like.

In one embodiment of the invention, the imide reaction as well as theevaporation of N-methyl-2-pyrolidone (NMP) is completed by heatingpolyamic acid on the surface of a metallic cylinder at about 120° C. for60 minutes; the imide reaction is completed halfway at about 120° C. for20 minutes of the heating treatment. NMP is the solvent contained in thepolyimide precursor solution of this embodiment. In proportion to thecoefficient of thermal expansion of the metal used for the metalliccylinder, the outside diameter of the metallic cylinder heated in anoven at about 200° C. expands. Accordingly, the inside diameter of thepolyimide tube expands and becomes the same as the outside diameter ofthe metallic cylinder. Then, by performing a cooling step, both theoutside diameter of the metallic cylinder and the inside diameter of thepolyimide tube shrink back to their size at room temperature. In otherwords, the polyimide tube keeps its adherence to the surface of themetallic cylinder after the cooling step. Even though the polyimideprecursor solution is a thermosetting resin, the solution which isheated to reach the middle stage of imide reaction can reduce its sizein accordance with the shift of temperature from 200° C. to roomtemperature. Therefore, since the polyimide tube is completely adheredto the metallic cylinder, primer solution with low viscosity can hardlypenetrate the gap between the outside surface of the metallic cylinderand the inside surface of the tube after dipping and holding the tube inthe primer solution. In this sense, treatments such as a maskingtreatment are not required for the metallic cylinder contacting theprimer solution and the edge of the tube in this dipping and holdingtreatment.

When the polyimide precursor solution is heated at 250° C. for 20minutes to reach the halfway imide reaction stage, the property of theprecursor solution as a thermosetting resin comes into effect.Therefore, even if the solution is cooled off down to room temperature,its inside diameter still stays equal to the outside diameter of themetallic cylinder at about 250° C. In this sense, while the outsidediameter of the metallic cylinder shrinks after the cooling step, theinside diameter of the tube stays the same, generating a clearancebetween the metallic cylinder and the tube. As a result, the primersolution permeates the gap between the cylinder and the tube afterdipping and holding the tube in the primer solution. Moreover, a similarphenomenon results if the primer solution is dried at a temperatureabove 200° C., and the dispersion permeates when the tube is dipped andheld in a fluororesin solution.

On the other hand, when the polyimide precursor solution is heated ataround 180° C. for twenty minutes, the progress of the imide reaction istoo slow to make a complete polyimide tube. After dipping and holdingthis incomplete polyimide tube in the primer solution, the tube, aidedby the water-absorptive property of NMP, becomes swollen with liquid. Asa result, the tube is likely to have wrinkles, and cannot providesignificant mechanical properties.

For these reasons, the temperature applied for heating and dryingtreatments of the invention is at a level that is the most suitable.However, the temperature level varies, depending on the materials of themetallic cylinders (coefficients of thermal expansion) and the kinds ofpolyimide precursor solutions and solvents.

In the method of manufacturing a polyimide composite tube of theinvention, the tube can be manufactured without separating the tube fromthe metallic tube in the middle of the manufacturing processes.Particularly, in order to provide additional functions, the tube can bemultilayered in this method.

The advantages of applying a dipping and holding treatment are asfollows:

(1) forming a tube with little loss in a coating solution compared witha spray-coating method;

(2) providing a flat surface for a tube;

(3) treating many cores at one time;

(4) providing high precision in the thickness of a tube; and

(5) easily arranging the thickness of a tube by changing the liftingspeed of a core and the viscosity of a solution.

The invention will now be explained specifically in the followingexample.

EXAMPLE

FIG. 1 is a flow diagram, showing manufacturing processes of a half-hardpolyimide layer of one embodiment. Letter A shows a process of coatingand molding a polyimide precursor solution on the surface of a metalliccylinder at a uniform thickness (casting process of steps A₁ -A₃); Bshows a process of drying the casted polyimide precursor solution(drying process); C shows a process of heating the solution to carry outthe imide reaction halfway (first heating process); D shows a coolingprocess.

FIG. 2 is a flow diagram, showing processes of coating fluororesin onthe surface of a seamless polyimide layer of the embodiment. Letter Eindicates a process of coating the primer; F is a process of drying theprimer; G is a cooling process; H shows a process of coatingfluororesin; I shows a process of heating the fluororesin to completethe imide reaction and bake the fluororesin (second heating process). Aprocess of removing a polyimide seamless composite tube is not shown inthese figures.

An aluminum cylinder 25 mm in outside diameter and 500 mm in length wasdipped and held in an inorganic coating solution containing siliconoxide, thereby coating the solution on the surface of the aluminumcylinder. The aluminum cylinder was heated and baked at 150° C. forthirty minutes and then at 350° C. for thirty minutes, thus preparing acore covered with silicon dioxide. The thickness of the silicon dioxidewas 2 μm, and its surface roughness measured by JIS-B0601 (Rz) was 0.8μm.

As shown in FIG. 1, a process of washing an aluminum cylinder (core) 1with water (a) and then drying the aluminum cylinder 1 with dry air (b)can be added to the casting process A. This process is useful foremploying the aluminum cylinder repeatedly. Then, a polyimide precursorsolution 3 is prepared by reacting both3,3',4,4'-biphenyltetracarboxylic acid di-anhydride and aromatic diamine(4,4'-diaminodiphenylether) in N-methyl-2-pyrolidone. After dipping andholding aluminum cylinder 1 in polyimide precursor solution 3 up to 400mm, aluminum cylinder 1 was lifted from the container containingpolyimide precursor solution 3. Then, an aluminum ring 2 (26 mm ininside diameter, 300 g in weight, and 45'in liquid contact angle) wasplaced on the top of the aluminum cylinder 1 coated with the polyimideprecursor solution, and slid down along the cylinder by its own weight.As a result, a polyimide precursor layer 0.5 mm in thickness was formedon the surface of aluminum cylinder 1.

In the drying process B, a casted polyimide precursor layer P₁ is driedin a drier 4 at 120° C. for sixty minutes. In order not to provide theeffect of hot air on polyimide precursor layer P₁, the layer was treatedvirtually without air movement.

In the first heating process C, the halfway imide reaction was promotedfor polyimide precursor layer P₁ in an oven 5 at 200° C. for 20 minutes,thus providing a half-hard polyimide layer P₂. The reduction ratio ofthe thickness of the polyimide precursor layer calculated by Formula 1was about 85%.

Half-hard polyimide layer P₂ was then cooled down to room temperature byblowing cold air (c) at the cooling process D. Without separatinghalf-hard polyimide layer P₂ from aluminum cylinder 1, the cylinder wastreated to the next procedures. Even though aluminum cylinder 1 wastreated individually up to these procedures, 10-100 cylinders werepreferably treated at one time in a pallet in the following processes.This composition is preferable for mass-production of a polyimidecomposite tube of the invention.

In the dipping process E, 50 half-hard polyimide layers P₂ held by agripping means 6 at the same time were dipped and held in a primercomposite solution 7 in which normal fluororesin primer (for example,Teflon 855-001 or Teflon 855-300 made by Dupont, and Polyflon EK-1700,Polyflon Ek-1800 or Polyflon 1900 made by Daikin Kogyo Sha; Teflon855-300 is used in this example) is mixed with 12% by weight of carbonblack powder. The viscosity of the primer was 80 c.p. In order to adhereprimer composite solution 7 to aluminum cylinder 1 at a uniformthickness, the cylinder was dipped and held in the solution whilerotating the cylinder at 1 r.p.m. After rotating the cylinder for 10seconds, aluminum cylinder 1 was lifted at 100 mm per minute. Instead ofrotating the cylinder, the cylinder can be vibrated up and down. As aresult, primer composite solution 7 was coated on aluminum cylinder 1evenly, while preventing a repelling phenomenon. In this way, primercomposite solution 7 can be mixed slowly at the same time.

Aluminum cylinder 1 coated with primer composite solution 7 was thenheated in a drier 8 at 180° C. for 30 minutes (drying process F). Afterheating primer composite solution 7, the solution was cooled down toroom temperature with cold air (d) during a cooling process G, therebyproviding P₃. (P₃ was a tube in which primer composite solution 7 wascoated on the surface of half-hard polyimide layer P₂.) The thickness ofthe primer layer after the drying process F and the cooling process Gwas 4 μm.

At the fluororesin coating process H, a solution 9 is prepared by adding0.6% by weight (relative to the weight of a solid in dispersion) ofKETJENBLACK (trade name: conductive carbon black invented by AKZO) intothe dispersion containing 45% by weight of fluororesin, composed of 70%by weight of polytetrafluoroethylene and 30% by weight of PFA. P₃ wasdipped and held in solution 9, thus coating the solution on the surfaceof the primer. The thickness of the layer made of solution 9 was about10 μm after drying the layer. The viscosity of the dispersion was 150c.p. Aluminum cylinder 1 was dipped and held in the dispersion for 10seconds while rotating the cylinder at 1 r.p.m. Then, the cylinder waslifted at 100 mm per minute.

The second heating process I for directing both the process ofcompleting the imide reaction and the process of baking a fluororesinwas then carried out. After heating aluminum cylinder 1 in an oven 10 at250° C. for 80 minutes, another 70 minutes heating treatment was appliedto the cylinder at 380° C. P₄ is a polyimide composite tube in which theprimer and the polytetrafluoroethylene were coated and baked on thesurface of a hard polyimide layer on aluminum cylinder 1. Polyimidecomposite tube P₄ was cooled in the cooling process J.

Finally, by taking off polyimide composite tube P₄ from aluminumcylinder 1, a seamless polyimide composite tube of the invention 25 mmin inside diameter and 350 mm in length was provided. The difference inthickness in longitudinal direction was ±1 μ.

FIG. 3 shows cross-sectional views of the seamless polyimide compositetube of the invention. 3 (a) shows a cross sectional view in thelongitudinal direction; 3 (b) shows a cross sectional view in thevertical direction (I--I). In FIG. 3, 11 is a polyimide layer(substrate); 12 is a conductive primer layer; 13 indicates apolytetrafluoroethylene layer; 14 shows an exposed area of a conductiveprimer layer. The surface electric resistance of conductive primer layer14 was 1×10⁵ Ω·cm.

FIG. 4, in addition, shows a perspective view of a seamless polyimidecomposite tube of the invention. Conductive primer layer 12 is coated onthe surface of polyimide layer 11; polytetrafluoroethylene layer 13 iscoated on the surface of conductive primer layer 12. In this example,one edge of conductive primer layer 12 was exposed at about 10 mm. Bycontacting a conductive brush, or the like to this exposed area 14,static, which is generated while the seamless polyimide composite tubeis running, can be discharged.

The seamless polyimide composite tube of the invention was applied forthe fixing apparatus of the electrophotographic printer of FIG. 5. Inother words, a live roller (22), a tension roller (23), and a heater(24) were located inside the polyimide composite tube; a backing-uproller was placed outside the tube of the invention. A copying paper(26) formed with toner (28) was supplied between seamless polyimidecomposite tube (21) and backing-up roller (25). Then, the toner wasfixed to the copying paper one after another by heater (24), therebyproviding fixed figures (28) on the copying paper. The seamlesspolyimide composite tube of the invention is so heat-resistant, strong,and anti-static that it is very useful. For example, when the polyimidecomposite tube of the invention was used for a laser printer, itwithstood the printing of about 100,000 sheets. V Moreover, thepolyimide composite tube of the present invention has a flat and smoothsurface of a baked fluororesin layer (outermost layer of the tube).Therefore, when the tube of the invention was used for a laser printer,it showed excellent running properties and properties of separatingtoner from itself.

The polyimide precursor solution used in the invention is prepared, forexample, by reacting aromatic tetracarboxylic acid and aromatic diamimein an organic polar solvent. For instance,3,3',4,4'-biphenyltetracarboxylic acid di-anhydride;2,3',4,4'-benzophenonetetracarboxylic acid di-anhydride; pyromelliticacid di-anhydride; or a mix of these tetracarboxylic acids can be usedas aromatic tetracarboxylic acid. However, the aromatic tetracarboxylicacid is not limited to these acids. Aromatic diamimes include,diphenylether diamimes such as 3,3'-diaminophenylether,3,3'-dimethoxy-4,4'-diaminodiphenylether, 4,4'-diaminophenylether andthe like; diphenylthioether diamimes such as 3,3'-diphenylthioether,4,4'-diaminodiphenylthioether and the like; benzophenone diamimes suchas 4, 4'-diaminobenzophenone and the like; m-phenylenediamime and thelike can be included; and the aromatic diamine is not restricted tothese diamines. N-methylpyrolidone, dimethylformamide,dimethylacetamide, phenol, o-cresol, m-cresol, p-cresol, dimethyloxideand the like are examples of suitable organic polar solvents. However,the organic polar solvent is not limited to these solvents.

Fluororesins include, for example, polytetrafluoroethylene (PTFE),tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin (PFA),tetrafluoroethylene-hexafluoropropylene copolymer resin (PEEP),ethylene-tetrafluoroethylene copolymer resin (PETFE),ethylene-chlorotrifluoroethylene copolymer resin (PECTFE),polyvinylidenefluoride (PVDF), or the like.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The embodimentsdisclosed in this application are to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

We claim:
 1. A polyimide composite tube for use to heat fixing belts inelectrophotographic printers, comprising a seamless cast thermosettingpolyimide layer as a substrate, a conductive primer layer on the surfaceof said polyimide layer, and a baked fluororesin layer on the surface ofsaid conductive primer layer.
 2. A polyimide composite tube according toclaim 1, wherein the seamless polyimide layer has a thickness of from 3μm to 500 μm.
 3. A polyimide composite tube according to claim 1,wherein the conductive primer layer comprises at least one compoundselected from the group consisting of polyphenylenesulfide,polyethersulfone, polysulfone, polyamideimide, polyimide, derivative ofpolyphenylenesulfide, derivative of polyethersulfone, derivative ofpolysulfone, derivative of polyamideimide, derivative of polyimide, andfluororesin.
 4. A polyimide composite tube according to claim 1, whereinthe conductive primer layer has a thickness of from 0.5 μm to 10 μm, andwherein said conductive primer layer comprises an exposed area.
 5. Apolyimide composite tube according to claim 1, wherein the conductiveprimer layer has a surface electrical resistance of from 1×10⁻² Ω·cm to1×10⁷ Ω·cm.
 6. A polyimide composite tube according to claim 1, whereinthe conductive primer layer comprises 1-40% by weight carbon powder. 7.A polyimide composite tube according to claim 1, wherein the fluororesinis at least one compound selected from the group consisting ofpolytetrafluoroethylene, tetrafluoroethylene-perfluoroalkylvinylethercopolymer, and tetrafluoroethylene-hexafluoropropylene copolymer.
 8. Apolyimide composite tube according to claim 1 wherein the fluororesinlayer has a thickness of from 2 μm to 30 μm.
 9. A polyimide compositetube according to claim 1, wherein the fluororesin layer comprises0.1-3.0% by weight carbon powder.